ada: Treat Info-Warnings as Info messages

ada: Treat Info-Warnings as Info messages

There was a general concept of info messages being a subset of
warnings. However that is no longer the case. Messages with an
info insertion character should be treated just as info messages.

gcc/ada/

	* atree.ads: Remove Warning_Info_Messages.
	* errout.adb: Remove various places where Warning_Info_Messages
	was used.
	* erroutc.adb: Remove various places where Warning_Info_Messages
	was used. Create Error_Msg_Object objects with only an info
	attribute if the message contained both info and warning insertion
	characters. New method Has_Switch_Tag for detecting if a message
	should have an error tag.
	* errutil.adb: Create Error_Msg_Object objects with only an info
	attribute if the message contained both info and warning insertion
	characters.